In high-speed optical fiber communication, a characteristic called chromatic dispersion restricts the transmission speed and transmission distance as well as causing deterioration in the optical waveform. This chromatic dispersion (hereafter called, dispersion) is a linear effect in which the different velocities of the signal propagating in the optical fiber are dependent on the wavelength. Strictly speaking, the optical waveform contains multiple waveform components (has a spreading spectrum) and when the group speed (different velocities) are wavelength dependent, slowly progressing (lag) components and quickly progressing (lead) components appear. These components cause the waveform to spread and when the dispersion value from this spread can no longer be ignored, distortion appears in the waveform causing the receive characteristics to deteriorate. The dispersion magnitude is proportional to the fiber length so this dispersion limits the transmission distance.
A dispersion compensator is generally utilized as a technique to avoid the effects of waveform distortion due to dispersion. The dispersion compensator is an optical device that provides dispersion that is an inverse quantity of the dispersion on the optical within the transmission path. This dispersion compensator can therefore cancel out the dispersion within the optical fiber and suppress waveform distortion due to dispersion. Dispersion compensation fiber (DCF) is most often used as the dispersion compensator. The structure and material of the dispersion compensation fiber are added so as to maintain dispersion characteristics that are opposite (opposite sign) those along the optical fiber's transmission path.
The dispersion compensation fiber is designed to cancel out the dispersion on the optical fiber (hereafter called, transmission fiber) of the transmission path. In single wavelength transmission for example, the dispersion compensation fiber is designed to provide a negative dispersion that is equivalent to the absolute dispersion value so as to compensate the dispersion on the transmission fiber signal wavelength.
However, the wavelength dependency of the dispersion must be taken into account when utilizing dispersion compensation fibers in WDM (wavelength division multiplexing) transmission. The transmission fiber dispersion value is usually larger on the long wavelength side, and can approximate a straight line on bands using WDM transmission. The dispersion (magnitude) per unit of distance, or in other words the straight line tilt when the dispersion coefficient (units: ps/nm/km) wavelength dependency approximates a straight line is called the dispersion slope (units: ps/nm/nm/km). The dispersion on the transmission fiber is a positive value, and for example in the dispersion-shifted fiber which is one of the main transmission fiber types, is generally a value of approximately 0.07 ps/nm/nm/km.
The dispersion along the transmission path becomes larger on the long wavelength side due to this dispersion slope, and in order to fully compensate for dispersion in WDM transmission where multiple channel signals are multiplexed along the direction of the wavelength, a DCF must be used whose dispersion compensation increases the longer the wavelength.
A method was for example disclosed for installing a dispersion compensation fiber 103 designed for compensating the dispersion slope as well as the dispersion, relative to the transmission fiber 102 as shown in FIG. 17 in JP-A No. 10(1998)-39155: Masashi Onishi et al., Invention, “Dispersion compensation fiber and Optical Dispersion Systems including same.” The interrelation of the transmission fiber 102 and dispersion compensation fiber 103 dispersion is shown in FIG. 18. The dispersion of the transmission fiber 102 reaches a dispersion (magnitude) of zero at the zero-dispersion wavelength (λ0), and from thereon increases towards the right shoulder. When the transmission band for sending the actual WDM signal is the wavelength λL (shortest wavelength) to the wavelength λU (longest wavelength), then the dispersion compensating fiber (103) is designed to equalize the dispersion absolute value along the signal band (from λL to λU), and further possesses a dispersion whose sign is negative, so as to completely cancel out the dispersion of the transmission fiber. The residual dispersion after this dispersion compensation is therefore zero along the signal band (from λL to λU).